There is currently much interest in developing thin-film circuits with TFTs and/or other semiconductor circuit elements on glass and on other inexpensive insulating substrates for large area electronics applications. Such TFTs fabricated with amorphous or polycrystalline semiconductor films may form the switching elements of a cell matrix, for example in a flat panel display as described in U.S. Pat. No. 5,130,829 (our reference PHB 33646), the whole contents of which are hereby incorporated herein as reference material. Recent developments involve the fabrication and integration of thin-film circuits (often using polycrystalline silicon) as, for example, integrated drive circuits for such a cell matrix. In order to increase the circuit speed, it is advantageous to use semiconductor material of good crystal quality and high mobility for the thin film bodies of the semiconductor circuit elements. In order to form such semiconductor material, it may be necessary to carry out processing steps at temperatures higher than those which the substrate can satisfactorily withstand.
U.S. Pat. No. 5,206,749 discloses a method of manufacturing an electronic device comprising a thin-film circuit having semiconductor circuit elements, which method comprises the steps of:
(a) depositing a thin film of material on a temporary support to provide a thin-film semiconductor body for the semiconductor circuit elements on the temporary support, and PA1 (b) transferring the thin-film semiconductor body from the temporary support to a substrate. PA1 (a) depositing a thin film of material on a temporary support to provide a thin-film semiconductor body for the semiconductor circuit elements on the temporary support, and PA1 (b) transferring the thin-film semiconductor body from the temporary support to a substrate, at least a part of which substrate is retained in the manufactured device, PA1 and which method in accordance with the present invention is characterised by the temporary support being a metal foil having opposite front and back faces which are unattached over most of their area, the thin-film semiconductor body is provided on the front face, and at least most of the metal foil is etched away by etching into the back face after the thin-film semiconductor body is secured to the substrate.
At least a part of this substrate is retained in the manufactured device and comprises electrically insulating material adjacent to the semiconductor thin-film bodies. It may be, for example, of a low temperature glass or a plastics material. By using a temporary support which is able to withstand higher temperatures than the substrate, the temperature restrictions on the formation of good quality semiconductor material for semiconductor thin-film bodies of the semiconductor circuit elements can be overcome. The whole contents of U.S. Pat. No. 5,206,749 are hereby incorporated herein as reference material.
U.S. Pat. No. 5,206,749 suggests adopting several measures in order to free the thin-film semiconductor body from the temporary support after their transfer to the substrate. Several of these measures involve depositing insulating material (e.g silicon dioxide or silicon nitride) on the temporary support before depositing the thin-film semiconductor body material, so as to provide a weakly-adhering or etchable release layer for the thin-film semiconductor body. However, in the case of a weakly-adhering release layer, it is difficult to cleave the release layer from the temporary support to which it requires good adhesion during the provision of the semiconductor body. In the case of an etchable release layer there can be a problem in providing adequate access for the etchant to the layer sandwiched between the substrate and the temporary support. As an alternative measure, U.S. Pat. No. 5,206,749 suggests attaching to the temporary support a releasable adhesive tape whose adhesion can be released by ultra-violet irradiation after securing the thin-film semiconductor body to the substrate.